1. Field of the Invention
The present invention relates to a wiper structure for vehicles and more particularly to a variable depressing pressure wiper structure wherein the depressing pressure can varied when the wiper blade is inverted its direction of movement.
2. Description of the Prior Art
In vehicle wiper structures such as those for a front windshield glass or a rear window glass, the wiper blade is reciprocated by means of a driving structure so that the direction of movement is periodically inverted. It has been experienced that the wiper blade hits the glass surface to produce noise when the direction of movement is inverted. It is therefore desired to decrease such noise produced when the wiper blade is inverted its direction of movement.
It has been recognized that the level of noise is mainly dependent on the pressure under which the wiper blade is depressed on the glass surface. It is therefore necessary to decrease the depressing pressure for decreasing the noise. The Japanese laid-open patent application Sho 62-59151 discloses a structure wherein the depressing pressure can be changed as desired. According to the proposal by the Japanese application, the wiper blade is supported on a wiper arm which is in turn supported on a rotatable shaft by means of a pivot pin for a movement toward and away from the glass surface. A spring is provided to force the wiper arm toward the glass surface so that the wiper blade is depressed on the glass surface. The spring is connected at one end to the wiper arm and at the other end with an adjustable member which is supported on a bracket or other type of stationary member in such a manner that the line of action of the spring is offset toward the glass surface from the line passing through the aforementioned one end of the spring and the aforementioned pivot pin supporting the wiper arm whereby the spring force produces a component directed toward the glass surface. A motor is provided to move the adjustable member at the end of the stroke of the reciprocating movement of the wiper blade so that the distance of the offset is decreased to thereby decrease the depressing pressure.
It should however be noted that the structure proposed by the Japanese patent application is not suitable for use in controlling the depressing pressure for the purpose of decreasing the noise produced when the movement of the wiper blade is inverted because it becomes necessary to detect the end of the stroke of the wiper movement.